1. Field of the Invention
The present invention relates to a passive seat belt device which automatically fastens and releases a seat belt by driving the seat belt, and more particularly to a seat belt device having means to protect a motor for driving the seat belt from an overload.
2. Related Background Art
In a passive seat belt device of the relevant type, an end of the belt is attached to a movable member, called a slider or runner, which is driven back and forth by a motor along a rail mounted to the vehicle body. The runner travels between a release position at a front or release end of the rail and a fasten position at a rear or fasten end of the rail. In such a device, the motor may be subject to overload and damage if, for example, the runner does not reach the fasten end (rear end) or release end an intermediate point of the rail. Devices intended to solve this problem are known in which a circuit breaker is inserted at an input of the motor or a timer is inserted in a drive circuit for the motor so that the motor is stopped a predetermined time after the application of a motor drive signal.
FIG. 1 shows a prior art circuit in which both the circuit breaker and the timer are provided in order to prevent the overload of the motor. SW.sub.D denotes a door switch, TM.sub.1 and TM.sub.2 denote timers, SW.sub.F and SW.sub.R denote limit switches respectively arranged at the front and rear ends of the rail, RL denotes a relay circuit, C/B denotes a circuit breaker and M denotes a motor.
When the vehicle door is opened, the door switch is thrown to an "open" position and the motor is rotated to drive the runner to the release (front) end of the rail. The limit switch SW.sub.F is arranged at the release end. Thus, when the runner reaches the release end, the switch SW.sub.F is turned off and the motor stops. When the door is closed, the runner is driven to the fasten end and the limit switch SW.sub.R at the fasten end is turned off so that the motor is stopped.
The drive device described above is expensive. However, use of the timers or circuit breaker alone can lead to problems. In the former case (timers only), if the runner stops between the ends of the rail and the door is opened and closed, forward and backward drive currents may be alternately supplied to the motor and the motor may be damaged. In the latter case (circuit breaker only), if the runner often stops at an intermediate position when a driver turns off the engine and leaves the vehicle, the motor is intermittently energized during a long time period and the vehicle battery is discharged.
Even if both the timer circuit and the circuit breaker are used as shown in FIG. 1, problems may still be encounterd. For example, let us assume that the runner stops halfway when it is driven forward but it smoothly moves backward. In such a case, the seat belt can fasten and secure an occupant as normal, but trouble may occur under the following condition.
When the door is opened under such a circumstance, the runner tends to move forward but it stops halfway. The circuit breaker is actuated before the timer TM.sub.1 times out, and the motor is deenergized. If the door is then closed, the circuit energizes the relay to drive the runner backward. However, since the circuit breaker is off, the motor does not rotate. If the circuit breaker is turned on within the set time of the timer TM.sub.2 and the runner is moved backward to the fasten position, no problem arises. However, if the timer first times out, the backward movement of the runner is blocked, and the runner is not moved to the rear end and the seat belt cannot fasten about the occupant, in spite of the fact that actual trouble is in moving the runner forward.
In this case, the set time of the timer TM.sub.2 for the backward movement of the runner may be extended, but there is a limit on the set time from the standpoint of prevention of damage to the motor. Thus, a combination of the circuit breaker and the timers does not work well.